A known crawler belt 51 for use in a construction machine or the like comprises assemblage of parts such as shown in FIG. 23. Among them, a track bushing 52 is required to have wear resistance at its inner and outer circumferential surfaces as well as high strength and toughness as a bushing, because it engages sprocket teeth for transmitting rotational movement from final reduction gears and functions to rotate the crawler belt 51. In a high speed running condition, there have been used pre-lubricated crawler belts containing a lubricant in a gap between a track pin 53 and the track bushing 52 in order to prevent occurrence of seizure between them. In this case, not only the wear resistance of the outer circumferential surface which comes into direct contact with the sprocket needs to be ensured, but also the lubricant must be sealed by a flat sealing portion 61 at an end face of the track bushing and a dust seal (lip seal) 62, and therefore, at least the contact area of the flat sealing portion 61 with which the dust seal 62 comes into contact is required to be sufficiently hardened by quenching (the contact area after wear is the area having a depth about one-half the wall thickness t from the outer circumferential surface).
To attain the required properties, any one of the following measures is usually taken in the production of such a bushing.
{circle around (1)} Case hardening steel is subjected to carburization treatment to form martensitic inner and outer surface layers having high hardness, whereby high wear resistance and strength are ensured (see Japanese Patent Publication (Kokoku) Gazette No. 52-34806).
{circle around (2)} The inner and outer circumferences of a refined bushing material, which has been prepared from medium carbon steel, are respectively subjected to induction hardening to form martensite of high hardness in the inner and outer surface layers. Alternatively, after deep quenching has been done by induction hardening from the outer circumferential surface, induction hardening is carried out from the inner circumferential surface to form a V-shaped tempered martensitic quench-hardened case between the outer and inner circumferential surface quench-hardened cases. With either of the above processes, desired wear resistance and strength are obtained (Japanese Patent Publication (Kokoku) Gazette No. 63-16314).
{circle around (3)} A bushing material (medium carbon steel) is once heated to a quenching temperature or more. After a specified time has elapsed after preceding cooling of the inner circumferential surface, cooling from the outer circumferential surface is stopped. Alternatively, the inner circumferential surface is cooled while heating the outer circumferential surface by induction heating and after an elapse of a specified time, the heating of the outer circumferential surface is stopped to start cooling of the outer circumferential surface. With such a series of quenching processes, quench-hardened cases respectively extending from the outer and inner circumferential surfaces toward the wall thickness center of the bushing are formed such that a smooth, U-shaped hardness distribution providing an unhardened quenched layer to be left between the quench-hardened cases is obtained and such that the hardened case formed in the outer circumferential surface is deeper than the hardened case formed in the inner circumferential surface. This provides a track bushing having excellent wear resistance and an inexpensive method of producing such a track bushing ((Japanese Patent Publication (Kokai) Gazette Nos. 11-61264 and 11-236619).
The carburization method {circle around (1)} produces a track bushing having good functionality as a pre-lubricated bushing because the end faces of the bushing are uniformly carburization hardened, but presents the problem that it takes a long time to carburize because the carburization-hardened case needs to be deepened in order to increase the wear resistance of the outer circumferential cylindrical surface, and use of a large amount of carburization gas leads to increased cost. In addition, where a large-sized track bushing having a great wall thickness is produced by the carburization method {circle around (1)}, the depth of the hardened case must be great in view of strength and wear resistance, which results in a decrease in yield and high cost. Further, since carburization heating of the inner and outer circumferential surfaces takes a long time, a grain boundary oxidation layer or imperfectly quenched layer of several tens of μm is formed in the inner and outer circumferential surfaces so that fatigue strength and impact resistance properties are likely to degrade.
The induction hardening method {circle around (2)} has been improved over the carburization method {circle around (1)} in terms of cost, but disadvantageously requires two quenching processes, i.e., quenching from the inner and outer circumferential surfaces, so that it cannot be regarded as a satisfactory thermal treatment having improved productivity and cost performance. Additionally, in this induction hardening method, irregularities and omissions in quenching of the end faces of the pre-lubricated track bushing are unavoidable and therefore there arises a problem in ensuring the wear resistance of the end face serving as a dust seal face for preventing penetration of earth and sand into the inner circumference section of the bushing.
The method {circle around (3)} in which the outer circumferential surface is cooled after preceding cooling of the inner circumferential surface, has the advantage that a track bushing excellent in wear resistance can be produced at very low cost by one quenching operation, but presents the drawback that if spray cooling is employed for cooling the inner circumferential surface of a cylindrical tube of small inside diameter, satisfactory cooling performance and uniformity cannot be ensured, because a surface to be cooled is usually positioned at a distance apart from the nozzle, the distance being about 10 times the bore diameter of the nozzle, in order to obtain an increased heat transfer coefficient and because the ability to discharge spray water is poor. Quenching irregularities in the inner circumferential surface are unavoidable particularly where the method is applied to a track bushing having a very small inside diameter or where, with the intention of attaining increased productivity, the inner circumferential surfaces of two or more track bushings are quenched with their end faces overlapping each other. As a result, a satisfactory improvement in productivity nor cost reduction cannot be achieved.
In the method disclosed in Japanese Patent Publication (Kokai) Gazette No. 11-236619 in which after a whole bushing material has been once heated, by induction heating from the outer circumferential surface, to a quenching temperature, preceding cooling of the inner circumferential surface is carried out while continuing heating of the outer circumferential surface, and after an elapse of a specified time, the heating of the outer circumferential surface is stopped to cool the outer circumferential surface, the time required for heating is longer than the time required for cooling and as heating speed increases, the outer circumferential surface of the track bushing is more heated than the inner circumferential surface, resulting in increased susceptibility to cracking at the time of quenching. This is the cause of hindrance to the realization of satisfactory productivity and stable quality. If heating is excessively done, it adversely affects the service life of the jigs for supporting the track bushing, because the jigs are also repeatedly cooled by the quenching medium after being heated.
Further, where the inner circumferential surface quenching is applied to two or more track bushings with their end faces overlapping each other as described earlier, a stable quench-hardened case cannot be formed in the flat sealing portions at both end faces of the track bushing because of the overlapping, so that this method cannot be applied to pre-lubricated bushings.
In addition, where spray cooling is adapted for cooling the inner circumferential surface, it is difficult to completely shut off a leakage of the cooling medium for the inner circumferential surface from the gap between each partition board and an end face of the bushing, the partition boards being in contact with both ends of the track bushing to separate water flows on the inner circumferential surface side and on the outer circumferential surface side from each other. Therefore, the above-described irregularities and omissions cannot be completely prevented in the quenched portions of the end faces of the pre-lubricated track bushing, so that there arises a need for an inspection for checking the wear resistance of the end faces which serve as the dust seal faces for preventing the penetration of earth and sand into the bore of the track bushing.
Even if a quench-hardened case can be formed with high productivity by induction hardening, a large-scale tempering furnace must be involved in the low temperature tempering process carried out subsequently to the quenching operation, which disadvantageously results in a large scale production line. In the case of induction tempering, tempering heating from the inner and outer circumferential surfaces becomes necessary, entailing unsatisfactory productivity.
The invention has been directed to overcoming the foregoing drawbacks and a primary object of the invention is therefore to provide a track bushing and its production method and apparatus, according to which inner circumferential surface cooling is carried out by an intratube laminar flow of a cooling medium so that a quenched case can be uniformly formed in the inner circumferential surface of a track bushing of small inside diameter or two or more track bushings being quenched in an overlapped manner, thereby achieving improvements over the carburization treatment and the induction quenching treatment in terms of productivity and cost.
Another object of the invention is to provide a track bushing production method and apparatus which are applicable to a pre-lubricated track bushing by virtue of the arrangement in which the inner circumferential surface of the bushing is cooled by an intratube laminar flow in order to completely shut off a leakage of the cooling medium for the inner circumferential surface from the gap between each end face of the track bushing and a partition jig for water-flow separation which is in contact with each end face of the bushing, and in which the speed of cooling from the inner circumferential surface in the neighborhood of both end faces of the bushing is slowed down by the partition jigs, thereby quench-hardening the flat sealing portions of the end faces.
The invention can obviate the need for heating from the inner circumferential surface and enables reliable quenching of the inner circumferential surface of a long cylindrical steel pipe of small inside diameter, and therefore another object of the invention is to provide a method and apparatus for producing, at low cost, a cylindrical part for general purpose which requires wear resistance and strength.
Still another object of the invention is to provide a method and apparatus for producing, at low cost, a track bushing having satisfactory toughness and wear resistance, in which a tempering treatment applied to a track bushing after quenching is eliminated or simplified. This method and apparatus are designed such that, in a series of quenching operations wherein preceding cooling from the inner circumferential surface is effected while carrying out induction heating from the outer circumferential surface and after an elapse of a specified time, cooling from the outer circumferential surface is carried out, a series of quenching processes is added in which during the additional induction heating from the outer circumferential surface or during the time from when the induction heating is stopped until when the outer circumferential surface cooling starts, the cooling from the inner circumferential surface is stopped for a specified time; the inner circumferential surface is tempered within a short time by heat diffusion from the outer circumferential surface to the inner circumferential surface; and the inner circumferential surface is cooled again, and after an elapse of a specified time, the cooling from the outer circumferential surface is carried out, whereby the quench-hardened inner circumferential surface layer is made to have a tempered martensitic structure, and whereby satisfactory toughness and wear resistance are achieved by eliminating the tempering of the quench-hardened outer circumferential surface layer or by effecting induction tempering from the outer circumferential surface within a short time.